<?php
function array_mer(){
    $result = [];
    $arrays = func_get_args();

    foreach ($arrays as $arr) {
        if (is_array($arr)){
            foreach ($arr as $val) {
                $result[] = $val;
            }
        }
    }
    return $result;
}
print_r(array_mer(['a', 'd'], ['a', 'b', 'c'], ['b', 'c']));